Magnetic ranging is a general term which is used to describe a variety of techniques which use magnetic measurements to determine the relative position (i.e., relative orientation and/or separation distance) of a borehole being drilled relative to a target such as another borehole or boreholes.
Determining the relative positions of two or more boreholes may be important in order to avoid intersection between boreholes, in order to achieve intersection of boreholes, or in order to achieve a desired relative position between boreholes.
It may be desirable to avoid intersection between boreholes when drilling a borehole in an area which is already crowded with other boreholes.
It may be desirable to achieve intersection between boreholes when drilling relief boreholes, drilling underground passages such as river crossings, or when linking a new borehole with a producing wellbore.
It is desirable to achieve a desired relative position between boreholes when employing steam assisted gravity drainage (SAGD) technology. SAGD technology involves the creation of a an upper borehole and a lower borehole which are essentially parallel to each other and are positioned essentially in a common vertical plane. The injection of steam into the upper borehole reduces the viscosity of hydrocarbons which are contained in the formations adjacent to the upper borehole and enables the hydrocarbons to flow due to gravity toward the lower borehole. The hydrocarbons may then be produced from the lower borehole using conventional production techniques.
In order for SAGD technology to be used effectively, careful control must be maintained during the creation of the pair of boreholes with respect to both the relative orientation of the boreholes and the separation distance between the boreholes. This control can be achieved using magnetic ranging techniques.
Magnetic ranging techniques include both “passive” techniques and “active” techniques. In both cases, the position of a borehole being drilled is compared with the position of a target such as a target borehole or some other reference such as ground surface. A discussion of both passive magnetic ranging techniques and active magnetic ranging techniques may be found in Grills, Tracy, “Magnetic Ranging Techniques for Drilling Steam Assisted Gravity Drainage Well Pairs and Unique Well Geometries—A Comparison of Technologies”, SPE/Petroleum Society of CIM/CHOA 79005, 2002.
Passive magnetic ranging techniques, sometimes referred to as magnetostatic techniques, typically involve the measurement of residual or remnant magnetism in a target borehole using a measurement device or devices which are placed in a borehole being drilled.
An advantage of passive magnetic ranging techniques is that they do not typically require access into the target borehole since the magnetic measurements are taken of the target borehole “as is”. One disadvantage of passive magnetic ranging techniques is that they do require relatively accurate knowledge of the local magnitude and direction of the earth's magnetic field, since the magnetic measurements which are taken represent a combination of the magnetism inherent in the target borehole and the local values of the earth's magnetic field. A second disadvantage of passive magnetic ranging techniques is that they do not provide for control over the magnetic fields which give rise to the magnetic measurements.
Active magnetic ranging techniques commonly involve the measurement, in one of a target borehole or a borehole being drilled, of one or more magnetic fields which are created in the other of the target borehole or the borehole being drilled.
A disadvantage of active magnetic ranging techniques is that they do typically require access into the target borehole in order either to create the magnetic field or fields or to make the magnetic measurements. One advantage of active magnetic ranging techniques is that they offer full control over the magnetic field or fields being created. Specifically, the magnitude and geometry of the magnetic field or fields can be controlled, and varying magnetic fields of desired frequencies can be created. A second advantage of active magnetic ranging techniques is that they do not typically require accurate knowledge of the local magnitude and direction of the earth's magnetic field because the influence of the earth's magnetic field can be cancelled or eliminated from the measurements of the created magnetic field or fields.
As a result, active magnetic ranging techniques are generally preferred where access into the target borehole is possible, since active magnetic ranging techniques have been found to be relatively reliable, robust and accurate.
One active magnetic ranging technique involves the use of a varying magnetic field source. The varying magnetic field source may be comprised of an electromagnet such as a solenoid which is driven by a varying electrical signal such as an alternating current in order to produce a varying magnetic field. Alternatively, the varying magnetic field source may be comprised of a permanent magnet which is rotated in order to generate a varying magnetic field.
In either case, the specific characteristics of the varying magnetic field enable the magnetic field to be distinguished from other magnetic influences which may be present due to residual magnetism in the borehole or due to the earth's magnetic field. In addition, the use of an alternating magnetic field in which the polarity of the magnetic field changes periodically facilitates the cancellation or elimination from measurements of constant magnetic field influences such as residual magnetism in the borehole or the earth's magnetic field.
The varying magnetic field may be generated in the target borehole, in which case the varying magnetic field is measured in the borehole being drilled. Alternatively, the varying magnetic field may be generated in the borehole being drilled, in which case the varying magnetic field is measured in the target borehole.
The varying magnetic field may be configured so that the “axis” of the magnetic field is in any orientation relative to the borehole. Typically, the varying magnetic field is configured so that the axis of the magnetic field is oriented either parallel to the borehole or perpendicular to the borehole.
U.S. Pat. No. 4,621,698 (Pittard et al) describes a percussion boring tool which includes a pair of coils mounted at the back end thereof. One of the coils produces a magnetic field parallel to the axis of the tool and the other of the coils produces a magnetic field transverse to the axis of the tool. The coils are intermittently excited by a low frequency generator. Two crossed sensor coils are positioned remote of the tool such that a line perpendicular to the axes of the sensor coils defines a boresite axis. The position of the tool relative to the boresite axis is determined using magnetic measurements obtained from the sensor coils of the magnetic fields produced by the coils mounted in the tool.
U.S. Pat. No. 5,002,137 (Dickinson et al) describes a percussive action mole including a mole head having a slant face, behind which slant face is mounted a transverse permanent magnet or an electromagnet. Rotation of the mole results in the generation of a varying magnetic field by the magnet, which varying magnetic field is measured at the ground surface by an arrangement of magnetometers in order to obtain magnetic measurements which are used to determine the position of the mole relative to the magnetometers.
U.S. Pat. No. 5,258,755 (Kuckes) describes a magnetic field guidance system for guiding a movable carrier such as a drill assembly with respect to a fixed target such as a target borehole. The system includes two varying magnetic field sources which are mounted within a drill collar in the drilling assembly so that the varying magnetic field sources can be inserted in a borehole being drilled. One of the varying magnetic field sources is a solenoid axially aligned with the drill collar which generates a varying magnetic field by being driven by an alternating electrical current. The other of the varying magnetic field sources is a permanent magnet which is mounted so as to be perpendicular to the axis of the drill collar and which rotates with the drill assembly to provide a varying magnetic field. The system further includes a three component fluxgate magnetometer which may be inserted in a target borehole in order to make magnetic measurements of the varying magnetic fields generated by the varying magnetic field sources. The position of the borehole being drilled relative to the target is determined by processing the magnetic measurements derived from the two varying magnetic field sources.
U.S. Pat. No. 5,589,775 (Kuckes) describes a method for determining the distance and direction from a first borehole to a second borehole which includes generating, by way of a rotating magnetic field source at a first location in the second borehole, an elliptically polarized magnetic field in the region of the first borehole. The method further includes positioning sensors at an observation point in the first borehole in order to make magnetic measurements of the varying magnetic field generated by the rotating magnetic field source. The magnetic field source is a permanent magnet which is mounted in a non-magnetic piece of drill pipe which is located in a drill assembly just behind the drill bit. The magnet is mounted in the drill pipe so that the north-south axis of the magnet is perpendicular to the axis of rotation of the drill bit. The distance and direction from the first borehole to the second borehole are determined by processing the magnetic measurements derived from the rotating magnetic field source.
In U.S. Pat. No. 4,621,698 (Pittard et al) the magnetic field sources are located at the back end or behind a percussion boring tool. In U.S. Pat. No. 5,002,137 (Dickinson et al) the magnetic field source is located within a percussive action mole.
In U.S. Pat. No. 5,258,755 (Kuckes) the magnetic field sources are located within a conventional drill collar which is behind a drill assembly which comprises a drill bit and a drilling motor. In U.S. Pat. No. 5,589,775 (Kuckes) the magnetic field source is located in a piece of drill pipe which is between a drill bit and a drilling motor.
There remains a need for a tool for connection into a drilling string which is adapted to perform at least one function in the drilling string in addition to providing and carrying a magnetic field source. In addition, there remains a need for such a tool for connection into a drilling string wherein the drilling string includes a rotary drill bit.